Inkjet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner into an image-recording element in response to digital signals. There are various methods which may be utilized to control the deposition of ink droplets on the receiver member to yield the desired image. In one process, known as drop-on-demand inkjet printing, individual droplets are ejected as needed on to the recording medium to form the desired image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation using heated actuators. With regard to heated actuators, a heater placed at a convenient location within the nozzle or at the nozzle opening heats ink in selected nozzles and causes a drop to be ejected to the recording medium in those nozzle selected in accordance with image data. With respect to piezo electric actuators, piezoelectric material is used in conjunction with each nozzle and this material possesses the property such that an electrical field when applied thereto induces mechanical stresses therein causing a drop to be selectively ejected from the nozzle selected for actuation. The image data provided as signals to the printhead determines which of the nozzles are to be selected for ejection of a respective drop from each nozzle at a particular pixel location on a receiver sheet. Some drop-on-demand inkjet printers described in the patent literature use both piezoelectric actuators and heated actuators.
In another process known as continuous inkjet printing, a continuous stream of droplets is discharged from each nozzle and deflected in an imagewise controlled manner onto respective pixel locations on the surface of the recording member, while some droplets are selectively caught and prevented from reaching the recording member. Inkjet printers have found broad applications across markets ranging from the desktop document and pictorial imaging to short run printing and industrial labeling.
A typical inkjet printer reproduces an image by ejecting small drops of ink from the printhead containing an array of spaced apart nozzles, and the ink drops land on a receiver medium (typically paper, coated paper, etc.) at selected pixel locations to form round ink dots. Normally, the drops are deposited with their respective dot centers on a rectilinear grid, i.e., a raster, with equal spacing in the horizontal and vertical directions. The inkjet printers may have the capability to either produce only dots of the same size or of variable size. Ink-jet printers with the latter capability are referred to as (multitone) or gray scale ink-jet printers because they can produce multiple density tones at each selected pixel location on the page.
Inkjet printers may also be distinguished as being either pagewidth printers or swath printers. Examples of pagewidth printers are described in U.S. Pat. Nos. 6,364,451 B1 and 6,454,378 B1. As noted in these patents, the term “pagewidth printhead” refers to a printhead having a printing zone that prints one line at a time on a page, the line being parallel either to a longer edge or a shorter edge of the page. The line is printed as a whole as the page moves past the printhead and the printhead is stationary, i.e. it does not raster or traverse the page. These printheads are characterized by having a very large number of nozzles. The referenced U.S. patents disclose that should any of the nozzles of one printhead be defective the printer may include a second printhead that is provided so that selected nozzles of the second printhead substitute for defective nozzles of the primary printhead.
Today the fabrication of pagewidth inkjet printheads is relatively complex and they have not gained a broad following. In addition there are problems associated with high-resolution printing in that simultaneous placement of ink drops adjacent to each other can create coalescence of the drops resulting in an image of relatively poor quality.
Swath printers on the other hand are quite popular and relatively inexpensive as they involve significantly fewer numbers of nozzles on the printhead. In addition in using swath printing and multiple passes to print an area during each pass, dot placement may be made selectively so that adjacent drops are not deposited simultaneously or substantially simultaneously on the receiver member. There are many techniques present in the prior art that described methods of increasing the time delay between printing adjacent dots using methods referred to as “interlacing”, “print masking”, or “multipass printing.” There are also techniques present in the prior art for reducing one-dimensional periodic artifacts or “bandings.” This is achieved by advancing in a slow-scan direction the paper or other receiver medium by an increment less than the printhead width, so that successive passes or swaths of the printhead overlap. The techniques of print masking and swath overlapping are typically combined. The term “print masking” generally means printing subsets of the image pixels in multiple passes of the printhead relative to a receiver medium. In swath printing a printhead, having a plurality of nozzles arranged in a row, is traversed in a fast-scan direction across a page to be printed. The traversal is such as to be perpendicular to the direction of arrangement of the row of nozzles.
With reference to commonly assigned U.S. Pat. No. 6,464,330 B1, filed in the names of Miller et al., an example of a printhead used in a swath printer is illustrated. The disclosure in this patent is incorporated herein by reference thereto. With reference to the accompanying FIG. 1, printhead 31 for each color of ink to be printed includes in this embodiment two printhead segments or modules or nozzle banks 39A and 39B. Each printhead nozzle bank includes two staggered rows of nozzles and the nozzles in each row of nozzles have a spacing of 1/150 inches between adjacent nozzles in the row. However, due to the presence of staggering there is a nominal nozzle pitch spacing, P, in each printhead nozzle bank of 1/300 inches as indicated in the figure. The nozzles on the second nozzle bank 39B are similar to that on the first nozzle bank 39A and the nozzle banks are arranged to continue the nozzle spacing for the printhead of 1/300 inches spacing between nozzles. The printhead nozzle banks may each also be referred to as a “nozzle module” because they are individually assembled into a supporting structure to form the printhead for printing a particular color. Each nozzle bank may also be referred to as a pen, segment or a module. Hereinafter, they will be referred to as a nozzle bank. It will be understood that for a printer having six different color inks, six printheads similar to that described for printhead 31 may be provided. The six different color printheads are arranged on a carriage that is traversed across the receiver sheet for a print pass. The nozzles in each of the six color printheads, are actuated to print with ink in their respective colors in accordance with image instructions received from a controller or image processor. Each printhead, would in the example of the subject printer, have two printhead nozzle banks.
To create pleasing printed images, the dots printed by one nozzle bank must be aligned such that dots printed by one of the nozzle banks are closely registered relative to the dots printed by the other nozzle banks jetting the same color ink. If they are not well registered, then the maximum density attainable by the printer will be compromised and banding artifacts will appear. Consider, for example a print made by a single color using a nozzle configuration as shown greatly magnified in FIG. 1, with two nozzle banks per color. As may be seen in FIG. 1, the two nozzle banks used to print each color are offset one from the other a predetermined known small distance “d” in the fast-scan direction. Such is a condition when proper registration is present and the printer adjusts the actuation or firing times of the nozzles in one nozzle tank to account for this small distance. If the nozzle banks are registered very well, it would be possible to print an image that appears as FIG. 2, with all of the paper covered by at least a single layer of ink. In this example, the image is hypothetically printed at 300 dpi using two banding passes per swath such that for printing a swath of pixels half of the dots are printed by the first nozzle bank and half of the dots are printed by the second nozzle bank. By contrast, if the two nozzle banks shown in FIG. 1 have a slight (˜35 micron) misregistration in the fast-scan direction, the dots do not properly align and some white space is generated as shown in FIG. 3. Likewise, if the misregistration is in the slow-scan direction, a similar situation occurs as shown in FIG. 4. Even more troublesome is a slight, relative skew between the two nozzle banks as shown in FIG. 5. In this case, at one end of the swath, good registration of the two nozzle banks is attained. At the other end of the swath, however, poor registration is incurred and banding is observed with a period equal to the height of the swath. Even very slight misalignments can result in objectionable image artifacts.
Large physical separations between two nozzle banks can make proper alignment even more difficult. Consider the nozzle bank arrangement as described in U.S. Pat. No. 4,593,295 by Matsufuji et al. To alleviate hue differences that may result from bi-directional printing, '295 teaches a particular arrangement of nozzle banks such that the ink order is symmetric with respect to an axis that is parallel to the slow-scan direction. To maintain this symmetry, one color of ink must be jetted by the leftmost nozzle bank(s) as well as by the rightmost nozzle bank(s) as shown in FIG. 4 of '295. In a typical inkjet printer, the distance between these nozzle banks may be 15 centimeters or more. Requiring precise alignment of two sets of nozzle banks being separated by such a distance is very challenging using typical techniques.
These are just some of the ways that the image quality produced by an inkjet printer can be compromised by poor registration of the various nozzle banks. Additionally, poor registration between the color planes can result in blurry or noisy images and overall loss of detail. These problems make good registration and alignment of all the nozzle banks within an inkjet printer critical to ensure good image quality. That is, not only should a nozzle bank be well registered with another that jets the same color ink, but it should also be well registered with nozzle banks that jet ink of another color.
In addition to good image quality, faster print rates are desired by customers of inkjet printers. For swath printers, a well-known means by which to accomplish high productivities is by increasing the number of nozzles. One way in which nozzle count may be increased is by simply adding extra nozzle banks. This has the advantage that the same print head design may be used. However, this adds to the number of nozzle banks that must be aligned, thereby increasing the possibility for misalignment and the labor required to properly align all the nozzle banks.
An alternative to gain higher productivity is to increase the nozzle count within a nozzle bank. This does not increase the count of nozzle banks, but usually results in longer nozzle banks as increasing the nozzle density of a nozzle bank typically requires a completely new print head design and/or a new manufacturing process. Longer nozzle banks also increase the difficulty of alignment of the nozzle banks as the sensitivity to angular displacements increases proportionately. For instance, the misregistration represented in FIG. 5 can result from a relative angular displacement of just 0.08 degrees if the two nozzle banks depicted in FIG. 1 are each one inch in length.
In high-end inkjet printers, such as one that might be used in a wide-format application, there are other considerations that must be made to ensure proper alignment of the nozzle banks. For instance, bi-directional printing in the fast-scan direction to increase productivity requires that the nozzle banks be properly aligned whether traveling in the right-to-left direction or the left-to-right direction.
Some high-end printers accept a variety of ink-receiving materials that may differ significantly in thickness. As a result, the printer may have several allowable discrete gaps between the nozzle banks and the printer platen to accommodate these different receivers. Invariably, the gap between the nozzle banks and the top of the receiver can vary significantly because of the range of receiver thicknesses and the limited number of discrete nozzle bank heights. Due to the carriage velocity, the flight path of the drop is not straight down but really is the vector sum of the drop velocity and carriage velocity. This angular path and the differences in nozzle bank heights make nozzle bank registration sensitive to both the average of the nozzle bank heights as well as the variation in nozzle bank heights. These sensitivities further complicate the nozzle bank alignment process.
Additionally, some high-end printers allow the customer to select different carriage velocities, higher carriage velocities resulting in increased productivity usually at a price in image quality. The term “carriage velocities” implies the supporting of the printheads upon a carriage support that moves in the fast-scan direction while being supported for movement by a rail or other support. The angular flight path of the droplets described will be a function of the carriage velocity. This then makes nozzle bank alignment sensitive to yet another variable, namely carriage velocity.
Yet another complicating factor is the use of multiple drop sizes of which many new print head designs are capable. As discussed above, the alignment of the printer is a function of the combination of the carriage velocity and droplet velocity. Due to differences in drag as the droplet flies through the air, different size droplets have different droplet velocities. Therefore, to provide good alignment, it may be desired to use different alignment settings for different drop sizes.
Current alignment techniques fall within two varieties. Visual techniques use patterns printed by the printer that permit a user to simultaneously view various alignment settings and chose the best setting (see, for example, U.S. Pat. Nos. 6,109,722 and 6,450,607). Visual techniques are disadvantaged in many ways. First, for a printer with many nozzle banks (24 separate nozzle banks is not uncommon), multiple print head heights, and multiple carriage velocities, the number of alignments can become overbearing as each variation adds multiplicatively to the rest. Secondly, only a moderate level of accuracy is attainable with most of these techniques and finely tuned printers require a higher degree of accuracy attainable by most of these techniques. The level of accuracy is further compromised between all color records by using a single color as the only reference. U.S. Pat. No. 6,450,607B1, for example, attempts to reduce this sensitivity by using the black nozzle bank as a reference for black and white images and a color nozzle bank when printing color images. For instance, a 4-color printer containing cyan, magenta, yellow and black may use cyan as the reference when printing color images. An accuracy of approximately 1/600th of an inch is quoted using the visual techniques described within U.S. Pat. No. 6,450,607B1 meaning that yellow and magenta may still be misregistered by two times 1/600th inch or 1/300th inch, despite practice of the invention disclosed by '607. Thirdly, interactions can occur between the various alignment parameters, which further degrade the ultimate quality of alignment that can be obtained through these visual techniques, or multiple iterations are required, thereby increasing the labor of the effort. Lastly, since several of these techniques usually operate by providing several alignment settings to the operator who then chooses the best choice, significant amounts of consumables (ink and media) may be required to obtain satisfactory alignment of all nozzle banks in all print modes.
The second way nozzle banks are typically aligned (e.g., U.S. Pat. Nos. 5,250,956, 6,478,401B1, and 5,451,990) is with an on-carriage optical sensor that interprets patterns printed by the nozzle banks to automatically make adjustments to the nozzle bank alignment. While much improved over the more common visual techniques, these methods, too, have several shortcomings. Firstly, they require additional hardware costs for each printer as a separate optical sensor and accompanying electronics are required. Secondly, the optical sensors are typically of the LED variety with economical optics and cannot provide the high degree of accuracy required of finely tuned, high-end printers. Thirdly, these sensors require significant averaging to create a reliable signal, making the amount of receiver required to perform the alignment larger than one would desire. Furthermore, this high degree averaging necessitates a separate measurement for each nozzle bank, requiring even more ink and receiver as the number of nozzle banks increases. Fourthly, these on-carriage optical sensors are typically arranged to provide data primarily in the fast-scan direction. For demanding applications, slow-scan adjustments are equally important. Some techniques provide means by which slow-scan misalignments may be determined, but these measurements require separate, additional patterns, further consuming additional ink and receiver. The patterns in U.S. Pat. No. 6,478,401B1, for example, require slanted blocks. The accuracy of the slow-scan measurement improves as the angle is made shallower, requiring additional receiver as greater accuracy is required. Furthermore, this fast-scan limitation makes determination of nozzle bank skew very difficult or impossible (U.S. Pat. No. 5,250,956, for example, requires 8 separate measurements to ascertain nozzle bank skew and U.S. Pat. No. 6,076,915 makes no provision for measurement of skew) and, as demonstrated in FIG. 5, this is a critical alignment dimension. Another result of the fast-scan directional limitation is the inability to measure errors in the advance of the receiver, yet another critical alignment variable. Lastly, these on-line optical sensor techniques have made no provision for alignment of a nozzle bank using different drop sizes wherein each drop size may optimally require slightly different alignments.
U.S. Pat. No. 6,347,857B1 implements an on-printer detection scheme by which single, isolated droplets are analyzed to ascertain the relative health of each nozzle so that corrective or compensating action may be taken in the case of poorly performing nozzles. To maintain rapid image capture for a relatively inexpensive device, the technique uses relatively low-cost capture techniques. While effective at detecting print head performance problems, it is incapable of detecting minute alignment errors shown to be detrimental in inkjet printing using multiple nozzle banks. Furthermore, no teachings of printed patterns capable of allowing such measurements are offered as part of the invention. Additionally, the invention disclosed in U.S. Pat. No. 6,347,857B1 requires additional printer hardware and special receiver for the analysis, adding to total printer cost.
It is therefore desired to develop a nozzle bank alignment technique and process that provides a high degree of accuracy of alignment of all critical alignment variables while requiring very little labor and time to execute and while consuming as little ink and receiver as possible.